The present invention relates to sequential thin film processing.
The fabrication of modern semiconductor workpiece structures has traditionally relied on plasma processing in a variety of operations such as etching and deposition. Plasma etching involves using chemically active atoms or energetic ions to remove material from a substrate. Deposition techniques employing plasma include Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) or sputtering. PVD uses a high vacuum apparatus and generated plasma that sputters atoms or clusters of atoms toward the surface of the wafer substrates. PVD is a line of sight deposition process that is more difficult to achieve conformal film deposition over complex topography such as deposition of a thin and uniform liner or barrier layer over the small trench or via of 0.13 μm or less, especially with high aspect ratio greater than 4:1. Plasma generation methods include parallel plate plasma, inductive coupled plasma (ICP), remote plasma, and microwave plasma. In parallel plate plasma, a power source is applied across two parallel plates to create an electric field which will ionize the gas to generate the plasma. The plasma is confined between the parallel plates where the electric field is strongest, and there is significant plasma bombardment due to the presence of the electric field. In inductive coupled plasma, a power source is applied to a coil to create a magnetic field which will ionize the gas to generate the plasma. A non-conducting window such as ceramic plate could be used to separate the plasma source from the plasma. Care should be taken so that no metal is deposited on the non-conducting window, otherwise the deposited metal will block the magnetic field, and the plasma will be extinguished. This is the reason why inductive coupled plasma was not used for metal deposition. Typical parallel plate plasma and inductive coupled plasma use radio frequency (RF) power sources. In remote plasma, a plasma is generated elsewhere and then being brought to the process chamber. In microwave plasma, the plasma uses microwave frequency (MW) power source. Microwave plasma tends to be remote plasma, and is brought to the process chamber using a microwave guide.
In CVD processing, a gas or vapor mixture is flowed over the wafer surface that is kept at an elevated temperature. Reactions then take place at the hot surface where deposition takes place. The temperature of the wafer surface is an important factor in CVD deposition; the temperature used depends on the chemistry of the precursor for deposition and affects the uniformity of deposition over the large wafer surface. CVD typically requires high temperature for deposition which may not be compatible with other processes in the semiconductor process. CVD at lower temperature tends to produce low quality films in terms of uniformity and impurities.
In a deposition technology that is similar to the CVD technique and is known as atomic layer deposition (ALD), various gases are injected into the chamber for as short as 100-500 milliseconds in alternating sequences. For example, a first gas is delivered into the chamber for about 500 milliseconds and the substrate is heated, thereafter the first gas (heat optional) is turned off. The residue from the first gas is then evacuated. Another gas is delivered into the chamber for another 500 milliseconds (heat optional). The residue from this gas is also evacuated before the next gas is delivered for about 500 milliseconds (and optionally heated). This sequence is complete when all gases have been cycled through the chamber; each gas sequence typically forms a monolayer which is highly conformal. ALD technology thus have pulses gas injection and heating sequences that are between 100 and 500 milliseconds. This approach has a high dissociation energy requirement to break the bonds in the various precursor gases such as silane and oxygen and thus requires the substrate to be heated to a high temperature, for example in the order of 600-800 degree Celsius for silane and oxygen processes.
ALD also uses radical generators, such as plasma generators, to increase the reactivity of the second gas and effectiveness the reaction between the first and the second gases at the substrate. U.S. Pat. No. 5,916,365 to Sherman entitled “Sequential chemical vapor deposition” provides for sequential chemical vapor deposition by employing a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. Sherman exposes a part to be coated to a gaseous first reactant, including a non-semiconductor element of the thin film to be formed, wherein the first reactant adsorbs on the part. The Sherman process produces sub-monolayer per gas injection due to adsorption. The first reactant forms a monolayer on the part to be coated (after multiple cycles), while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer but is available to react with the deposited monolayer. A pump removes the excess second reactant and reaction products thus completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.
There are other applications that use plasma in ALD processes. U.S. Pat. No. 6,200,893 to Sneh entitled “Radical-assisted sequential CVD” discusses a method for CVD deposition on a substrate wherein radical species are used in alternate steps to depositions from a molecular precursor to treat the material deposited from the molecular precursor and to prepare the substrate surface with a reactive chemical in preparation for the next molecular precursor step. By repetitive cycles a composite integrated film is produced. In a preferred embodiment the depositions from the molecular precursor are metals, and the radicals in the alternate steps are used to remove the ligands left from the metal precursor reactions, and to oxidize or nitride the metal surface in subsequent layers.
In one embodiment taught by Sneh, a metal is deposited on a substrate surface in a deposition chamber by (a) depositing a monolayer of metal on the substrate surface by flowing a molecular precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with an inert gas; (d) flowing at least one radical species into the chamber and over the surface, the radical species is highly reactive with the surface ligands of the metal precursor layer and eliminates the ligands as reaction product, and saturates the surface, providing the first reactive species; and (e) repeating the steps in order until a metallic film of desired thickness results.
In another Sneh aspect, a metal nitride is deposited on a substrate surface in a deposition chamber by (a) depositing a monolayer of metal on the substrate surface by flowing a metal precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing a first radical species into the chamber and over the surface, the atomic species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product and also saturating the surface; (e) flowing radical nitrogen into the chamber to combine with the metal monolayer deposited in step (a), forming a nitride of the metal; (f) flowing a third radical species into the chamber terminating the surface with the first reactive species in preparation for a next metal deposition step; and (g) repeating the steps in order until a composite film of desired thickness results.
The Sneh embodiments thus deposit monolayers, one at a time. This process is relatively time-consuming as a thick film is desired.
Another application of sequential deposition is nanolayer thick film deposition (NLD) which is disclosed in U.S. patent application Ser. No. 09/954,244 on Sep. 10, 2001 Nguyen et al. NLD is a process of depositing a thin film by chemical vapor deposition, including the steps of evacuating a chamber of gases, exposing a workpiece to a gaseous first reactant, wherein the first reactant deposits on the workpiece to form the thin film, then evacuating the chamber of gases, and exposing the workpiece, coated with the first reactant, to a gaseous second reactant under plasma, wherein the thin film deposited by the first reactant is treated to form the same materials or a different material.
In comparison with CVD, atomic layer deposition (ALD or ALCVD) is a modified CVD process that is temperature sensitive and flux independent. ALD is based on self-limiting surface reactions. ALD provides a uniform deposition over complex topography and temperature independent since the gases are adsorbed onto the surface at lower temperature than CVD because it is in adsorption regime.
As discussed in Sherman and Sneh, the ALD process includes cycles of flowing gas reactant into the chamber, adsorbing one sub-monolayer onto the wafer surface, purging the gas reactant, flowing a second gas reactant into the chamber, and reacting the second gas reactant with the first gas reactant to form a monolayer on the wafer substrate. Thick film is achieved by deposition through multiple cycles.
Precise thickness can be controlled by regulating number of cycles used since a monolayer is deposited per cycle. However, the conventional ALD method is slow in depositing films such as those around 100 angstroms in thickness. Growth rate of ALD TiN for example was reported at 0.2 angstrom/cycle, which is typical of metal nitrides from corresponding chlorides and NH3.
The throughput in workpiece fabrication for a conventional ALD system is slow. Even if the chamber is designed with minimal volume, the throughput is still slow due to the large number of cycles required to achieve the thickness. The pump/purge cycle between gases is very time consuming, especially with liquid or solid vapors. Conventional ALD is a slower process than that of CVD with a rate of deposition that is almost 10 times as slow as CVD deposition. The process is also chemical dependent to have the proper self-limiting surface reaction for deposition. To improve the throughput, a batch system has been developed to process many wafers at the same time.
As with other sequential processing methods, the precursor gases or vapors are introduced sequentially with a pump/purge step in-between to ensure the complete removal of the precursor. This pump/purge step does not contribute to the film process, therefore it is desirable to eliminate this step from the processing sequence.